1. Field of the Invention
The present invention relates to a copolymer for the lithography used in production of semiconductor and a process for producing the copolymer. More particularly, the present invention relates to a copolymer for the semiconductor lithography suited for fine processing using a radiation such as far ultraviolet radiation, X-rays, electron beam or the like, as well as to a process for producing the copolymer.
2. Description of the Prior Art
In the lithography employed for semiconductor production, formation of a finer pattern is necessary with an increase in integration degree. In the formation of a fine pattern, a light source of short wavelength is essential. Currently, a lithography using a Krypton fluoride (KrF) excimer laser beam (wavelength: 248 nm) is the main lithography used in semiconductor mass production and, also, a lithography using an argon fluoride (ArF) excimer laser beam (wavelength; 193 nm) is being introduced in semiconductor mass production. Further, lithographies using a fluorine dimer (F2) excimer laser beam (wavelength: 157 nm), extreme ultraviolet radiation (EUV), X-rays, electron beam or the like are being developed.
The resist polymers used in these lithographies are constituted so as to contain the following recurring units:
a recurring unit having a structure wherein a polar group soluble in alkaline developing solution (the group is hereinafter referred to as alkali-soluble group) has been protected with an acid-dissociating, non-polar substituent group for suppressing the solubility in alkaline developing solution (the substituent group is hereinafter referred to as acid-dissociating, dissolution-suppressing group),
a recurring unit having a polar group for increasing the adhesivity to semiconductor substrate or the like, and, as necessary,
a recurring unit having a polar or non-polar substituent group for controlling the solubility in resist solvent or alkaline developing solution. In the lithography using a KrF excimer laser beam as the light source, there are known copolymers containing a recurring unit derived from hydroxystyrene, and a recurring unit wherein a phenolic hydroxyl group derived from hydroxystyrene has been protected with an acid-dissociating, dissolution-suppressing group or a recurring unit wherein a carboxyl group derived from (meth)acrylic acid has been protected with an acid-dissociating, dissolution-suppressing group or the like (reference is made to, for example, Patent Literatures 1 to 4). There are also known copolymers containing a recurring unit using an alicyclic hydrocarbon group as an acid-dissociating, dissolution-suppressing group for increasing the dry etching resistance and the dissolution contrast between exposed area and non-exposed area (reference is made to, for example, Patent Literatures 5 to 6).
In the lithography using an ArF excimer laser beam of shorter wavelength or the like as the light source, there was investigated a copolymer containing no recurring unit derived from hydroxystyrene having a high absorptivity coefficient to a wavelength of 193 nm. As a result, there are known copolymers having, in the recurring unit, a lactone structure as a polar group for increasing the adhesivity to semiconductor substrate or the like (reference is made to, or example, Patent Literatures 7 to 10), and copolymers having, in the recurring unit, a polar group-containing, alicyclic hydrocarbon group (reference is made to, for example, Patent Literature 11). With these copolymers, however, there arose a problem of generation of pattern defect after development and there have been made various studies for improvement. As one such improvement, there are known copolymers having a lactone structure and an alicyclic hydrocarbon group as recurring units and further containing a recurring unit derived from (meth)acrylic acid (reference is made to, for example, Patent Literatures 12 to 13).
These techniques can suppress development defect; however, they were not sufficient in lithography properties such as DOF (depth of focus) and the like. Thus, there have been strongly needed a resist polymer small in development defect and superior in lithography properties and a process for producing such a resist polymer.
Patent Literature 1: JP-A-1984-045439
Patent Literature 2: JP-A-1993-113667
Patent Literature 3: JP-A-1998-026828
Patent Literature 4: JP-A-1987-115440
Patent Literature 5: JP-A-1997-073173
Patent Literature 6: JP-A-1998-161313
Patent Literature 7: JP-A-1997-090637
Patent Literature 8: JP-A-1998-207069
Patent Literature 9: JP-A-2000-026446
Patent Literature 10: JP-A-2001-242627
Patent Literature 11: JP-A-1999-109632
Patent Literature 12: JP-A-2000-321771
Patent Literature 13: JP-A-2000-338673